Photosensitization can result when light interacts with endogenous or exogenous chemical agents in the skin and eyes. This process can produce undesirable clinical consequences, as in phototoxicity and photoallergy; or it can have beneficial effects, as in tumor photodynamic therapy (PDT) and coal-tar or psoralen (PUVA) therapy against psoriasis. Photosensitization results from the light-induced production of free radicals and/or singlet oxygen 1-O-2 , the lowest electronic excited state of molecular oxygen. Because the latter species may be important in both phototoxic reactions and PDT, we have developed state-of-the-art instrumentation capable of detecting the characteristic phosphorescence of 1-O-2 at 1268 nm. This instrumentation has permitted us to delineate the photophysics of 1-O-2 production from a number of photosensitizers including phenothiazines, tetracyclines, benzoxazoles synthetic dyes, anthralin and 1,8-dihydroxyanthraquinone. The major component of Disperse blue 35 (a dye that causes photodermatitis in factory workers) was identified as 4,5-diamino-1,8-dihydroxyanthraquinone and shown to be an efficient generator of 1-O-2. Singlet oxygen was also implicated in the phototoxicity of benzanthrone (7H-benz[de]anthracene-7-one), a dye intermediate prepared from 1,8-dihydroxyanthraquinone. A nanosecond flash photolysis spectrometer has been built and is being tested. This equipment will permit us to carry out time-resolved transient absorption and emission spectroscopy on excited state intermediates (precursors to 1-O-2) of photosensitizers.